[truth1ness]

Well click dragging a folder in is one click drag onto the Calibre icon then one click to pick add method (and one can get to the folder via spotlight with no clicks or directly from desktop or browser downloads area).

Without it it's click on the calibre icon to open it, right click add, left click correct add option, then manually navigate the file explorer which could be ~1-4 clicks (depending where it was last), click on target folder, click Add.

So it's 2-3 clicks versus 5-10 clicks. Not terrible but for a fairly common task I'd say worth streamlining if it's not a huge effort. It also makes the behavior consistent for files and folders which is generally good UI design.